1. Technical Field
The present invention relates to a drive system. More particularly, the present invention relates to a drive system for an illumination device.
2. Related Art
An encoding manner of a pulse width modulation (PWM) is generally adopted in the illumination of LEDs to control a light density, color coordinates and a color temperature of the LEDs, instead of driving the LEDs in a manner changing the current or voltage.
As for the PWM, an ON/OFF ratio of a pulse is adjusted during one cycle to determine an average output power. The longer the ON time during one cycle is, the greater the average output power is, and the brighter the LEDs are. However, the general control manners of the PWM have the following characteristics. (1) One LED drive unit has switched (ON or OFF) for many times during one cycle, and the electromagnetic interference (EMI) increases in a proportion of a square of the switching times of the LED drive unit. (2) When the PWM controls a plurality of LED drive units, the worst situation at the power supply end occurs when all the LED drive units are switched from OFF to ON or from ON to OFF simultaneously. In this case, the current to be changed simultaneously in such a short time is a sum of the currents of all the LED drive units. When the system includes a greater number of LED drive units, the instant value of the total current is rather large, and as a result, the EMI is increased. Specifically, if the power current is switched for many times during one cycle, the EMI is increased severely.
Furthermore, a great current change rate may result in a power rail-collapse at the power supply end, and as a result, the power supply voltage received by the integrated circuit (IC) may be insufficient, and thus the IC cannot work normally.
In addition, if the Z-profile of a circuit is not well designed by a circuit designer for the power supply end and the ground end, the great current change rate may result in an overshoot of the power supply voltage. If the overshoot voltage extent exceeds the maximum value an IC in that circuit can endure, the IC may be damaged.
Furthermore, the rising time of the power may not come up with the great current change rate, and thus cannot come up with the rapid state transition. Therefore, the power integrity (PI) problem should be considered.
In the U.S. Pat. No. 4,845,481, a design of driving a plurality of LEDs in a manner of PWM encoding scheme has been proposed, in which the brightness of the LEDs is controlled by manipulating the driving power's pulse width.
In the U.S. Pat. Nos. 6,016,038, and 7,113,541, LEDs are controlled by processor in a PWM way, but a current switching problem still exits. In the U.S. Pat. No. 6,132,061, the PWM current switching problem is solved by using a dummy load. However, as for a plurality of LED drive units, at the instant of a low load or no loads, all the large currents flow through the dummy load, and such large currents result in a great energy waste at the dummy load, which produces a lot of heats, and does not meet the energy saving requirement. Furthermore, as for the power supply, there are still switching ripples, which may bring interferences to the circuit.